A CAS (Chemical Abstracts Service) Registry Number is a unique identifier that tells you, for example, that acetone and dimethyl ketone are actually the same substance. From a safety and inventory perspective, this is a terrific idea. As of March 2007, there were 31,000,000 unique substances indexed by CAS. See http://www.cas.org/faq.html for more info. The Chemical Abstracts Service, http://www.cas.org/, is a division of the American Chemical Society, http://www.acs.org/.